20 Vertebrate Embryology 



can be followed as far as the thirty-two-cell 

 stage, irregularities often being seen as early 

 as the four-cell stage. 



As early as the eight-cell stage, sections of 



A and S, early and late stages in the segmentation of the egg. C, 

 beginning of the archenteron, Ar. (gastrulation). S C, segmentation- 

 cavity (camera lucida ; C slightly altered). 



the egg (Fig. 5, IT) show a small central cavity, 

 the segmentation cavity, which becomes larger 

 as segmentation proceeds, and is filled with an 

 albuminous fluid. This cavity, as will be seen, 



